The current disclosure is intended to document further improvements discovered subsequent to submission of provisional patent requests 61/436,160, 61/479,111, 61/485,319 and U.S. 61/509,004 which are all herein incorporated by references. Specifically, modifications disclosed herein include a screw head, of approximately the same size as screw heads in the current art, and transferring load from Belleville style spring washers used as spring elements in bone screws. Bone screws as disclosed herein may be fabricated without self-tapping sharp tips. Installation tapping bits can be used to create gaps on the engaged side of threads, such that in the fully tensioned or compressed state, the screw threads engage the bone more uniformly.
The process of bone fixation includes reapproximating two osseous structures and then holding them together while bone healing takes place. However, within 2 to 24 hours after bone surgery, screws installed to hold fractured bone together loosen significantly.
Often, loose screws will back out of the bone, causing soft tissue irritation. With significant loosening, bone fragments become subject to movement at the fracture site. This movement creates many problems during the healing process including further injury, prolonged healing, nonunion and/or an inappropriate union. These disadvantages often cause recommendation for prolonged non-weight bearing, with pain and with potential for abuse of pain medication. The risks of prolonged non-weight bearing are Deep Vein Thrombosis (DVT's), osteopenia changes, muscle atrophy, joint ankylosis and generalized soft tissue contractures including ligaments, tendons and neurovascular bundles. Depending on the patient and the time frame that non-weight bearing is necessary, some of the aforementioned risks may be non-reversible resulting in chronic morbidity.
Studies have shown that by increasing weight bearing earlier, there is not only a significant reduction in the chronic morbidities listed above, but there is also an increase in functionality and an earlier return to activities. The complications with early weight bearing are the possibility of current hardware failure and fatigue, fracture fragments shifting due to inadequate compression or non-anatomic fixation, and non-unions stemming from either an avascular compromise or micromotion in a seemingly anatomic correction.
The goal of a dynamic compression screw is to allow continued compression that can withstand the forces of early protected weight bearing. This would reduce a substantial amount of physician bills and insurance payouts that can occur due to complications and lost employment.
Screws are routinely installed with the self-tapping, sharp and pointed tip extending beyond the far outside bone surface, into the soft tissues. The disclosed bone hole that is tapped for screw installation, by the conventional screw thread tip, has negative value. The screw tips used to be very dull and not as pointed until the older art and industry standard was altered in an attempt to be self-tapping and thereby speed up the surgical process. Screws now have a bone cutting tip that is usually tricar in nature and very sharp in an attempt to reduce the tapping step. The problem is that the pointed and sharp screws need to be installed with their threads typically extending 2-3 threads beyond the cortex, i.e., into soft tissues. But doing so leaves a sharp knife like object that is exposed outside the bone in soft tissues. This exposed object can create trauma to surrounding tendons, ligaments, bone periosteum and can increase long term post-operative pain.
Finite Element Analysis (FEA) and experimental observations have shown that the highest screw pullout stresses occurred at the upper thread location. Research suggests screws stretch significantly when loaded in tension or shorten when loaded under compression. Either condition accumulates strain toward an end where the maximum strain concentrates stress at the last single screw thread in the bone. The traditional result is the potential for a progressive failure mode wherein a failed thread transfers the highest strain and stress to end threads, which in-turn may overstress bone. Overstressed bone results in microscopic fatigue. The body reacts by resorption causing screw loosening at typical installation loads. The resorption continues down the thread, effectively shearing the screw loose from the bone. Full screw thread to bone engagement is not achieved in the current art. The disclosed device suggests pre-tapping and overdrilling the hole, as mentioned above, and eliminating the sharp cutting tip.
Further, the present device, may also integrate a Belleville washer single spring, stacked spring or reversing stacked spring section into the head of the screws to enable load bearing on the relatively thin but strong cortical bone. This enables the screws to retract and continue to provide the necessary pressure on the bone to hold fracture sites together, secured against movement from internal and external forces, and under compression specific to promoting bone growth. This increases rates at which the bone will heal, decreases soft tissue damage from traditional screw lifting, and prevents additional injury, delayed union, nonunion, and/or inappropriate union. Using these improvements to the current art, fixation can more frequently be done internally, eliminating a source for pin-tract infection risk from external fixation and substantially reducing the complications mentioned above.
Previously engineered dynamic screws are inadequate. Other configurations are simply not structurally capable of producing the tension necessary to hold the bones together at the fracture site, are too rigid to provide sufficient retraction during the loosening phase of healing, or both.
Without these spring screws, animals are often euthanized as the only humane option, and people spend a lot of time in various types of recovery that will no longer be necessary. These new dynamic bone screws enable internal fixation of fractures or osteotomy sites. Internal fixation can improve by closing the fracture zone gap that naturally occurs during healing, and maintain consistent rigid fixation. The proposed screws do not loosen, but relax, stabilizing the joint while allowing earlier movement and weight bearing to stimulate bone growth—reducing net healing time. Traditional head lifting is eliminated, with its attendant soft tissue irritation, screw breakage and the need for surgical removal or retightening. An osteotomy or fracture under the resulting dynamic stabilizing forces, allows early, often immediate motion and weight bearing, as tolerated, resulting in significantly earlier rehabilitation and return to normal activity.
The devices herein disclosed would allow for minimally invasive procedures while allowing patients a quicker return to activity with potential weight bearing immediately after surgery as tolerated, without increasing the risk of nonunion. An earlier return to normal activity and less frustration for patients and their doctors is expected with the device. For a veterinarian, repairing a complex leg fracture on a horse, the ability to achieve weight bearing immediately or very soon after internal fixation surgery can be the difference between life and death for the animal.
The spring screws and cable connections disclosed herein, are mentioned in provisional patents incorporated herein by reference 61/436,160; 61/479,111; 61/485,319; and U.S. 61/509,004; and contain very small, but strong, nested, stiff springs, designed to perform dynamically during internally and externally applied forces and following bone retraction, from healing and from screw bearing-induced bone retraction. The screws and cables are thereby capable of sustained delivery of the level of bone compression sufficient to trigger bone growth and resist internal and external forces while contracting to accommodate the live bone reaction to internal fixation. The devices disclosed herein can retain pressure in a very specific range of bone stresses necessary to trigger bone growth, avoid progressive fissuring from stress concentrations above yield, and avoid stress shielding of the bone.
Creating a blunt screw tip, as suggested in the incorporated provisional applications, can fully engage bone with the full shank of threads. This enables the screw to grab the far cortex after drilling through the near cortex, adhering to the cortical rim while compression takes place. This constitutes a new advancement in the art.
When an osteotomy or fracture is reduced for fixation, whether natural or cut, at the micro scale, the bone-bone interface only actually touches initially at a few small contact points. Screw landings and threads also initially bear on the bone only at small contact points. These small initial contact points, in the fracture zone, below landings and on top of screw threads are locally highly stressed by hardware that is producing compression in the osteotomy. The contact points fissure and are soon resorbed by the body. Early resorption also results in progressive diminution of the fracture gap. Diminution is the gradual (2 to 24 hours) reduction in the physical dimension between cut or fracture surfaces during the natural healing process. Tension in rigid hardware (used to create compression in an osteotomy or natural fracture) is reduced by diminution of the cut or natural fracture gap. An orthopedic screw, installed through a fractured joint, stretches a few microns when it is tightened to a specified torque. When diminution occurs in the joint, the stretching relaxes and the installed force of tension in the screw is correspondingly relaxed. Further relaxation of hardware may result from continued fissuring and resorption of small contact points under landings and on top of threads bearing on bone. This fissuring diminution may continue until a more uniformly distributed stress at bone to bone contact points and on head & thread to bone contact areas achieve a net bone yield stress levels of 60 N/mm2 or below, seating the joint and hardware. With conventional hardware, tension in the screw drops quickly, because the screws are stiff.
By integrating a spring-like section into screws and cables, proposed herein, the hardware will retract and continue to provide the necessary pressure on the bone for diminution and seating to complete while continuing to hold fracture sites together under compression. Internal fixation, using such spring loaded hardware, is secured against movement from internal and external forces, and can be optimized to sustain compression specific to promoting bone growth. It is disclosed that this may enable bone bridging, increasing rates at which the bone will heal, decrease soft tissue damage from traditional screw lifting, and prevent additional injury, delayed union, nonunion, and/or malunion. Using such dynamic hardware, fixation can more frequently be done internally, eliminating external fixation sources for pin-tract infection.
It is herein postulated that achieving a more uniformly distributed stress distribution between threads and bone could achieve a net higher pullout value and quickly seat the hardware at bone yield stress levels below 60 N/mm2, above which natural fissuring and resorption would otherwise continue to occur. By limiting bone fissuring and resorption on active bearing thread surfaces, less spring action will be required to maintain a tight, functional screw.
The device more efficiently utilizes the cross section of the purchased bone, and delivers a significantly higher level of compression, reliably sustained within the desired range to promote bone growth throughout the natural bone retraction that occurs during the healing process following fixation which the current art fails to accomplish. Existing competitive screws have sharp, pointed tips whereas the disclosed device may not include sharp, pointed tips.
Installation tapping bits can create gradually tapered gaps in the active engagement zone, e.g., on tension screws, above threads, such that in the fully tensioned state, the screw threads engage the bone more uniformly. By matching fully stressed screw dimensions with the pre-threaded gap used to install it, the threads more uniformly engage the bone, yielding a substantially increased pullout value.